1. Field of the Invention
This invention relates to a method for producing a wafer for backside illumination type solid imaging device, and more particularly to a method for producing a wafer for backside illumination type solid imaging device used in mobile phones, digital video cameras and the like.
2. Description of the Related Art
Recently, a high-performance solid imaging device using a semiconductor is mounted onto a mobile phone, a digital video camera or the like. As the performance to be required in the solid imaging device are high-quality pixels and ability of taking moving images, and in order to take moving images, it is required to combine a high-speed computing device with a memory device, and hence a CMOS image sensor allowing System on Chip (SoC) easily is used and the downsizing of the CMOS image sensor is developed.
With the downsizing of the CMOS image sensor, however, there is caused a problem that an aperture ratio of a photodiode as a photoelectric conversion device is necessarily reduced to lower a quantum efficiency of the photoelectric conversion device, which makes it difficult to improve S/N ratio of imaging data. Therefore, it is attempted to conduct a method for increasing incident light quantity by inserting an inner lens into a front side of the photoelectric conversion device, or the like. However, the remarkable improvement of S/N ratio can not be realized.
In order to increase the incident light quantity to improve the S/N ratio of the image data, therefore, it is attempted to feed the incident light from a backside of the photoelectric conversion device. The greatest merit of the light incidence from the backside of the device lies in a point that there is no restriction in the reflection or diffraction on the front face of the device or the light receiving area of the device as compared with the light incidence from the front side. On the other hand, when the light is entered from the backside, the light absorption through a silicon wafer as a substrate for the photoelectric conversion device must be suppressed, and hence the thickness of the solid imaging device as a whole is required to be less than 50 μm. As a result, the working and handling of the solid imaging device become difficult, causing a problem of extremely low productivity.
For the purpose of overcoming the above technical problems, there are mentioned backside illumination type solid imaging devices as disclosed, for example, in JP-A-2007-13089 and JP-A-2007-59755.
In JP-A-2007-13089 is disclosed a method for producing a solid imaging device, which allows the production of a backside illumination type CMOS solid imaging device having a structure that electrodes are taken out from a surface opposite to an illuminated surface relatively simply and easily since a semiconductor substrate is thinned after bonding with a support substrate to ensure a strength and a through-hole interconnection is formed after the thinning of the support substrate.
In JP-A-2007-59755 is disclosed a solid imaging apparatus wherein internal stress and strain of a semiconductor substrate can be made small but also the processing of a color filter, a micro-lens or the like onto a thinned surface of a semiconductor substrate can be conducted in a high accuracy as well as a production method thereof.
In the solid imaging devices of these patent documents, however, the gettering ability of the substrate (wafer) is low, so that there are problems that white defects occur and that heavy metal contamination occurs in the production process. Therefore, it is required to solve these problems in order to put the backside illumination type solid imaging device into practical use.